MDEV-7950 Item_func::type() takes 0.26% in OLTP RO

Step 2c:

After discussion with Igor, it appeared that Item_field and Item_ref
could not appear in this context in the old function build_equal_item_for_cond:

  else if (cond->type() == Item::FUNC_ITEM ||
           cond->real_item()->type() == Item::FIELD_ITEM)

The part of the condition checking for Item_field::FIELD_ITEM was a dead code.
- Moving implementation of Item_ident_or_func_or_sum::build_equal_items()
to Item_func::build_equal_items()
- Restoring deriving of Item_ident and Item_sum_or_func from Item_result_field.
  Removing Item_ident_or_func_or_sum.

sql/item.cc

@@ -734,7 +734,7 @@ Item_ident::Item_ident(TABLE_LIST *view_arg, const char *field_name_arg)
 */
 
 Item_ident::Item_ident(THD *thd, Item_ident *item)
-  :Item_ident_or_func_or_sum(thd, item),
+  :Item_result_field(thd, item),
    orig_db_name(item->orig_db_name),
    orig_table_name(item->orig_table_name), 
    orig_field_name(item->orig_field_name),

sql/item.h

@@ -2134,19 +2134,7 @@ public:
 };
 
 
-class Item_ident_or_func_or_sum: public Item_result_field
-{
-public:
-  Item_ident_or_func_or_sum(): Item_result_field() { }
-  Item_ident_or_func_or_sum(THD *thd, Item_ident_or_func_or_sum *item)
-    :Item_result_field(thd, item)
-  { }
-  COND *build_equal_items(THD *thd, COND_EQUAL *inherited,
-                          bool link_item_fields);
-};
-
-
-class Item_ident :public Item_ident_or_func_or_sum
+class Item_ident :public Item_result_field
 {
 protected:
   /* 
@@ -2336,6 +2324,25 @@ public:
   {
     update_table_bitmaps();
   }
+  COND *build_equal_items(THD *thd, COND_EQUAL *inherited,
+                                  bool link_item_fields)
+  {
+    /*
+      normilize_cond() replaced all conditions of type
+         WHERE/HAVING field
+      to:
+        WHERE/HAVING field<>0
+      By the time of a build_equal_items() call, all such conditions should
+      already be replaced. No Item_field are possible.
+      Note, some Item_field derivants are still possible.
+      Item_insert_value:
+        SELECT * FROM t1 WHERE VALUES(a);
+      Item_default_value:
+        SELECT * FROM t1 WHERE DEFAULT(a);
+    */
+    DBUG_ASSERT(type() != FIELD_ITEM);
+    return Item_ident::build_equal_items(thd, inherited, link_item_fields);
+  }
   bool is_result_field() { return false; }
   void set_result_field(Field *field) {}
   void save_in_result_field(bool no_conversions) { }
@@ -3459,7 +3466,7 @@ public:
   An abstract class representing common features of
   regular functions and aggregate functions.
 */
-class Item_func_or_sum: public Item_ident_or_func_or_sum, public Item_args
+class Item_func_or_sum: public Item_result_field, public Item_args
 {
 public:
   Item_func_or_sum() :Item_args() {}
@@ -3471,7 +3478,7 @@ public:
   Item_func_or_sum(Item *a, Item *b, Item *c, Item *d, Item *e)
     :Item_args(a, b, c, d, e) { }
   Item_func_or_sum(THD *thd, Item_func_or_sum *item)
-    :Item_ident_or_func_or_sum(thd, item), Item_args(thd, item) { }
+    :Item_result_field(thd, item), Item_args(thd, item) { }
   Item_func_or_sum(List<Item> &list) :Item_args(list) { }
   bool walk(Item_processor processor, bool walk_subquery, uchar *arg)
   {
@@ -3576,20 +3583,18 @@ public:
   table_map used_tables() const;		
   void update_used_tables(); 
   COND *build_equal_items(THD *thd, COND_EQUAL *inherited,
-                          bool link_item_fields)
+                                  bool link_item_fields)
   {
     /*
-      Item_ref cannot refer to Item_field when build_equal_items() is called,
-      because all "WHERE/HAVING field" are already replaced to
-      "WHERE/HAVING field<>0" by this time. See normalize_cond().
-      TODO: make sure with Igor and Sanja, perhaps the below expression
-      can be simplified just to a Item::build_equal_items() call.
+      normilize_cond() replaced all conditions of type
+         WHERE/HAVING field
+      to:
+        WHERE/HAVING field<>0
+      By the time of a build_equal_items() call, all such conditions should
+      already be replaced. No Item_ref referencing to Item_field are possible.
     */
-    return real_type() == FIELD_ITEM ?
-           Item_ident_or_func_or_sum::build_equal_items(thd, inherited,
-                                                        link_item_fields) :
-           Item::build_equal_items(thd, inherited,
-                                   link_item_fields);
+    DBUG_ASSERT(real_type() != FIELD_ITEM);
+    return Item_ident::build_equal_items(thd, inherited, link_item_fields);
   }
   bool const_item() const 
   {

sql/item_func.h

@@ -131,6 +131,8 @@ public:
     used_tables_and_const_cache_init();
     used_tables_and_const_cache_update_and_join(arg_count, args);
   }
+  COND *build_equal_items(THD *thd, COND_EQUAL *inherited,
+                          bool link_item_fields);
   bool eq(const Item *item, bool binary_cmp) const;
   virtual optimize_type select_optimize() const { return OPTIMIZE_NONE; }
   virtual bool have_rev_func() const { return 0; }

sql/sql_select.cc

@@ -12893,9 +12893,9 @@ COND *Item_cond::build_equal_items(THD *thd,
 }
 
 
-COND *Item_ident_or_func_or_sum::build_equal_items(THD *thd,
-                                                   COND_EQUAL *inherited,
-                                                   bool link_item_fields)
+COND *Item_func::build_equal_items(THD *thd,
+                                   COND_EQUAL *inherited,
+                                   bool link_item_fields)
 {
   COND_EQUAL cond_equal;
   cond_equal.upper_levels= inherited;